Paint Calculator

Gallons = ceil( ( length × height − openings ) × coats ÷ coverage per gallon )

L

Total length of the walls to be painted, in feet

H

Ceiling height, in feet

A

Area of doors and windows left unpainted, in square feet

n

Number of coats of finish paint

c

Coverage of one gallon of that paint, from the can, in square feet

Use it once you know which paint you are buying and have the can in front of you, because the coverage figure is the one input that cannot be looked up reliably. The two decisions that move the answer are the coverage (an interior matte and an exterior paint can differ by two to one) and the number of coats, which multiplies the whole area rather than adding to it. Measure the perimeter of the room, or add the four walls together, rather than measuring one wall and multiplying — the walls are rarely the same length once a chimney breast, a cupboard or a bay window is in the way. Deduct the doors and windows only if you are not painting the frames and the reveals: a roller takes the wall up to the frame, and the area inside the frame is usually a fifth of what people expect.

1. A 12 ft by 15 ft bedroom, two coats, 350 ft² per gallon

   1. Wall area: 54 × 8 = 432 square feet, less 32 square feet of door and window = 400 square feet
   2. Two coats: 400 × 2 = 800 square feet of surface to cover (74.32 m²)
   3. Gallons: 800 ÷ 350 = 2.29, rounded up to 3 gallons
   4. Litres: 2.29 × 3.785 = 8.67, rounded up to 9 litres

   These are the page defaults, and the two outputs are the interesting part: the bare arithmetic says 2.29 gallons, which rounds up to 3, and 2.29 gallons is also 8.67 litres, which rounds up to 9. Three US gallons is 11.4 litres — more than the 9 litres on the row above — because a whole gallon is a coarser package than a whole litre. Both rows are correct for the market they are sold in.

2. A 20 ft by 9 ft garage wall, one coat, 400 ft² per gallon

   1. Wall area: 20 × 9 = 180 square feet (16.72 m²), with no openings to deduct
   2. One coat, so the surface to cover stays 180 square feet
   3. Gallons: 180 ÷ 400 = 0.45, rounded up to 1 gallon
   4. Litres: 0.45 × 3.785 = 1.70, rounded up to 2 litres

   A quarter of a gallon is not a product, so one coat on one wall still means buying a whole can — and a part-used can is worth keeping, because the second coat over a patched section will need the same batch. Note how far the coverage figure moves the answer: at 250 square feet per gallon this wall would still take one gallon, and at 150 it would take two.

3. A 120 ft run of exterior masonry, two coats, 150 ft² per gallon

   1. Wall area: 120 × 10 = 1,200 square feet, less 200 square feet of windows and doors = 1,000 square feet
   2. Two coats: 1,000 × 2 = 2,000 square feet (185.81 m²)
   3. Gallons: 2,000 ÷ 150 = 13.33, rounded up to 14 gallons (two and a bit five-gallon pails)
   4. Litres: 13.33 × 3.785 = 50.47, rounded up to 51 litres

   A textured masonry wall swallows paint: the same 2,000 square feet that takes 6 gallons of interior matte takes 14 gallons here, and the difference is entirely the surface rather than the wall. This is the example that shows why the coverage is an input — an exterior job that is estimated from an interior coverage figure is out by more than a factor of two.

The area arithmetic is exact; the coverage figure is a magnitude, not a specification. The default of 350 square feet per gallon is a typical interior matte, and real paints run from about 150 square feet a gallon on rough stucco or brick to 400 on a smooth sealed surface — the same paint on a bare new plaster wall can lose a third of its coverage, and spraying puts more on the wall than rolling. So the number printed on the can always beats the number on this page, and the coverage field exists so that you can type it in. Three things that also need paint are deliberately not counted here: primer, which is a separate product with its own coverage (about 250 square feet a gallon, and a bare or patched wall needs it); filler and caulk, which are bought by the tube rather than by the gallon; and the ceiling, which is a separate length × width calculation — run the page again with the room's width as the wall length. Masking tape, drop sheets, roller sleeves and brushes are not paint and are not counted either. Finally, the litre figure is a conversion of the gallon figure rather than an independent estimate: it is exact to the millilitre, but the rounding to whole litres is not, and no supplier sells a can sized to your answer.

How much paint do I need for a 12 by 15 foot room?

Three gallons, if you are painting 400 square feet of wall with two coats of a paint that covers 350 square feet a gallon: 400 × 2 = 800 square feet, and 800 ÷ 350 = 2.29, which rounds up to 3. That is also 8.67 litres, which is 9 whole litres. A room of that size is the page default, so you can press the button and see the arithmetic change as you move the coverage figure.

Should I deduct the doors and windows?

Deduct the glass, not the frame. A roller takes the wall up to the frame and a brush takes the frame itself, so an opening of 32 square feet — one door and one window — is worth about 32 square feet of wall. The reveals and the frame are extra, and on a room with several windows the deduction is a smaller share of the total than most people expect. Leave the field at zero if you are painting everything inside the frame as well.

How many coats should I allow for?

Two is the normal answer for a colour change or a fresh wall, and one only works over an identical colour on a sealed surface. Three is normal when going from a dark colour to a light one, and it is worth knowing that the second coat over a sealed surface covers further than the first, which is soaking into the wall — so a two-coat job is not exactly twice the paint of a one-coat job, but the difference is not something to rely on when ordering.

Where do I find the coverage per gallon?

On the can, usually as a range in square feet or square metres per gallon, and often split between a smooth surface and a rough one. That range is the honest answer: 350 square feet per gallon is normal for an interior matte, 400 for a satin over a sealed wall, 250 for a primer and 150 for masonry paint on stucco. Take the lower end of the range printed on your can if the wall is bare, textured or has not been painted before.

My paint is sold in litres, not gallons

Multiply the figure on the can by 3.785 to get the coverage per gallon, because the field is a per-gallon figure — a can that covers 10 square metres a litre covers about 37.9 square metres a gallon. The reference table lists both the per-gallon and the per-litre figure for five common paints, and the litre output is rounded up to whole litres, which is the package you would actually buy.

Does this include the primer, the ceiling and the trim?

No. Primer is a separate product with its own coverage — budget about 250 square feet a gallon and remember that a bare, patched or dark wall needs it. The ceiling is a separate calculation: run the page again with the room's width as the wall length and the room's length as the height. Doors, skirting, architraves and window frames are usually painted in a different product, which is why the openings are deducted rather than counted separately.

• Approximate Conversions from U.S. Customary Measures to Metric — the chart behind gallons, litres and square feet — National Institute of Standards and Technology
• The International System of Units (SI) brochure — the litre and the square metre the outputs are converted to — Bureau International des Poids et Mesures (BIPM)